"Our shop stewards will picket at the nine provincial hearings that the National Energy Regulator of South Africa will hold between now and the end of January," Numsa secretary Irvin Jim said in Durban on Sunday.
If Eskom's application succeeded, it would result in job losses and general price increases, he said.
Numsa had conducted a snap electricity cost survey among 10 companies with energy-intensive production processes in November.
It found that 37.5% of them anticipated closing down as a result of the proposed increase.
"Even more distressing is the fact that all but one company stated they would have to retrench workers as a result of the proposed tariff increase," said Jim.
"The companies further stated that the job losses would range between 300 and 600 per company in the five year period."
Increase effects
The increase would lead to higher prices of food and other consumer goods, he said.
Jim added that municipalities would also hike their prices, which would have a negative impact on local economies and would destroy local jobs.
The increase in electricity prices would also destroy any chances of dealing with the challenges of unemployment, inequality and poverty, he said.
Jim said Eskom should consider inflation-related and not double-digit increases.
"Through the increases, consumers and customers are asked to pay for Eskom's inefficiency."
Jim said that investigations by National Energy Regulator of South Africa and the Public Protector into load-shedding had revealed that one of the causes of the imposed blackouts was Eskom's failure to secure coal contracts on time.
They had also been delays in Eskom's build programmes and projects. – Sapa